The present invention relates to a sealed passage for a negative terminal through a metal wall, wherein the sealing between said terminal and the metal wall is provided by a glass seal.
Such passages are to be found in particular in electrochemical cells, and especially in cells having a negative electrode based on lithium. This is because of the highly reactive nature of the electrolytes used which makes it essential to provide a perfectly sealed container for the cell with a seal that performs properly at all the temperatures to which the cell is likely to be exposed.
However, it has been observed in lithium cells, regardless of the positive active material associated with the lithium (copper oxide, silver chromate, thionyl chloride, . . . ) that during discharge the glass of the seal in said passage through the metal wall is degraded.
This degradation takes place according to an electrochemical process which makes use of the oxides that constitute the glass, cations of lithium in solution in the electrolyte, and electrons due to the lithium electrode dissolving.
For SiO.sub.2, the principal constituent of glass, the process may be written as follows: EQU SiO.sub.2 +4Li.sup.+ +4e.sup.- .fwdarw.2Li.sub.2 O+Si
it will be understood that this type of reaction can be extended to all the other constitutents of glass, with the reaction rate differing from one constituent to another.
Proposals have already been made to slow down or stop the degradation of the glass by modifying the composition of conventional glasses or by insulating the glass from the electrolyte by a deposit of macromolecular resins or of metallic oxides which are stable under the operating conditions of such a cell.
However these solutions are onerous and not very reliable.
Preferred embodiments of the present invention remedy these drawbacks.